To improve health care, there has been considerable effort with regard to the administration of intravenous (IV) fluids. Both controllers and pumps have been developed for delivering metered amounts of IV fluid to the patient.
A variety of systems have been utilized to supply or administer various liquids such as blood, nutrient or pharmaceutical solutions, and so on to human and animal patients. When intravenous administration of liquids is desired, the most commonly used apparatus to achieve such administration comprises a container for the liquid to be administered, a tube connected to the container, and a hollow needle or plastic catheter at the end of the tube to be introduced into the patient's vein, with the fluid flowing under gravity out of the container through the tube. Frequently, some manually operated mechanical device is provided, such as an adjustable clamp, for controlling the rate of flow from the storage container into the patient by varying the resistance in the tube to the fluid flow. The actual flow rate is dependent in addition on the pressure of the fluid passing through the tube, which is in turn a function of the differential in height between the level of liquid in the container and the point of administration to the patient, or externally applied pressure sources.
In the above described gravity systems, the rate of flow into the patient, i.e., the quantity of liquid administered to the patient per unit time, is subject to substantial fluctuation. Even though a transparent drip chamber is frequently provided in these gravity systems whereby the rate of drops flowing from the container into the tube can be observed and measured per unit time, the actual rate of fluid outflow from the administering system and into the patient is quite variable. Variation in rate of flow can be the result of different degrees of resistance in the system from differences in fluid density and viscosity, or even of variations in back pressure exerted against the fluid flow from changes in the patient's blood pressure. Moreover, the volume of the drips is neither constant nor precise. Accordingly, the number of drops is not a precise indication of flow rate. Such fluctuations in flow rate experienced with the widely-used gravity systems can lead to undesirable consequences.
Improved systems for the administration of liquids to patients have been proposed in the prior art to overcome some of the drawbacks of the gravity-based systems. These improved systems are essentially of two types. In the first type, an attempt is made to provide means for controlling the resistance to flow through the system in a more accurate and refined manner than the standard adjustable clamp which constricts the tube through which the fluid flows. For example, mechanical variable resistance devices have been interposed in the fluid flow line which do not require constriction or crimping of the tubing but instead provide flow-through apertures of various sizes, depending on the degree of resistance desired. A second type of improved prior art device attempts to regulate by a variety of means the pressure head of the fluid flowing from the container or bag.
One common apparatus comprises a stand from which is suspended a liquid reservoir, e.g., in the form of a bag made of plastic material. Liquid is fed under gravity from the reservoir to the patient via a tube which is formed with a drip chamber. A clamp is provided downstream of the drip chamber, and can be adjusted so as to vary the flow restriction caused by the clamp, and thus vary the flow rate. The flow rate is estimated by counting the rate of drips in the drip chamber.
Setting up the apparatus is time consuming, and requires priming of the drip chamber which, if not done carefully, could result in problems such as air bubbles in the liquid being delivered to the patient. Also, in some circumstances the apparatus is not sufficiently accurate. In these cases, the tendency is to use a peristaltic pump for liquid infusion. This gives highly accurate results, but is very expensive, and requires special calibrated tubing which must be changed daily.
Another known form of apparatus uses a sensor for counting drips and a regulator to control flow in response to the drip rate. Commonly this is accomplished by means of a disposable cassette. This also has the problem of the drip chamber cassettes making the cost of usage high, and has a fairly high current consumption.
The rate at which an intravenous fluid is to be administered to a patient depends upon such factors as the particular kind of operation to be performed on the patient, the seriousness of the patient's illness or injury, or the patient's pulse rate, blood pressure or heart condition. For example, 500 milliliters of intravenous fluid are usually administered in 1 to 3 hours, but are sometimes administered in 4 or 5 hours.
During most normal procedures, IV fluids are administered continuously over extended periods of time at relatively low flow rates. Oftentimes, however, a situation, such as the need for surgery or fluid resuscitation, will arise where a continuous low flow fluid path will not satisfy the needs of the patient. Under these conditions, the low-flow administration set-up is removed and replaced with a high-flow set-up. When the patient's special needs are satisfied, the high-flow administration set-up is removed and once against replaced with a new low-flow set-up.
This repeated setting up and taking down of the IV system is a time consuming procedure which wastes substantial amounts of health care time. The loss of time, particularly during emergency procedures, can increase the patient's risk factor. In fight of the fact that an IV administration set-up can be used only once, the use of multiple set-ups during a single procedure can be relatively costly. Further, inventory levels must take into account the need for multiple set-ups, and multiple set-ups means multiple disposal as well. Frequent IV starts increase the risk of infection to the patient. Thus, any reduction in the number of starts and set-ups used per patient will be of an immediate benefit to both patient and health care workers alike.
Presently available IV sets provide either low-flow controlled flow rates, or high flow rates that are poorly controlled. At present, the end user must switch back and forth between these two types of sets. High flow rate sets can be dangerous, as it is difficult to control the flow rate when low rates are desirable. In addition, it is difficult to accurately monitor the flow rate under such conditions.
A variety of fluid flow meters have been described. See, for example, U.S. Pat. No. 4,389,901, and references cited therein.
Sacco (U.S. Pat. No. 5,059,173) has disclosed a dual chamber IV set, one chamber having a "mini" drip, and one a "maxi" drip. Thus Sacco envisions two drip chambers: no chamber is "dripless". While Sacco discloses a "high-flow" limb, he is not addressing flow rates necessary for volume resuscitation, but rather flow rates that are merely higher than available with a "mini" drip device. At flow rates necessary for volume resuscitation, the drip chamber of the Sacco "high-flow" limb would be a source of bubbles which could lead to air emboli. When fluid is forced through the "high-flow" limb of the Sacco device and into the "maxi" drip chamber, the fluid is entering the chamber as a high-speed jet. A Venturi effect results, causing air to entrain in the fluid stream, which creates great turbulence and aeration in the chamber and frothing of the liquid. A train of air bubbles can thereby be drawn into the administration line and then into the flow device and on into the patient. Such bubbles are a fife-threatening hazard to the patient. In contrast, the subject invention has a "dripless" chamber which eliminates the possibility of air emboli. Also, the subject invention uses a flow indicator component rather than a drip chamber to indicate flow rate.